In microelectromechanical (MEMS) sensors such as tuning fork gyroscopes, accelerometers, and other sensors, a proof mass is supported above a dielectric (e.g., glass) substrate and motion of the proof mass is sensed electronically.
In one MEMS design for a tuning fork gyroscope, two proof masses are suspended above a substrate by support flexures anchored to the substrate. Adjacent the outer sides of each proof mass is a drive electrode structure above the substrate in the same plane as the proof masses and with electrode fingers interleaved with electrode fingers of the proof mass in a comb-like geometry. Between and adjacent the two proof masses and also in the same plane as the proof masses is a pick-off or sense electrode structure also with electrode fingers interleaved with electrode fingers of the proof mass. Beneath each proof mass on the substrate is a sense plate.
The drive electrodes oscillate the proof masses electrostatically in the plane of the device. In response to an inertial input, the proof masses deflect out of the plane of vibration and this deflection is detected by the pick-off electrode. See U.S. Pat. No. 5,747,961 by the assignee hereof hereby incorporated herein by this reference.
Typically, a DC bias voltage is applied to the sense plates on the substrate beneath the proof mass and another DC bias voltage is applied to the drive and pick-off electrode structures adjacent the proof masses to simplify the design of the electronic signal processing circuitry and to maintain precise phase relationships.
Any exposed dielectric substrate beneath the proof masses ultimately assumes a steady state voltage level between the DC bias voltage applied to the sense plates and the DC bias voltage applied to the drive electrodes. Reaching that steady state voltage level, however, can take hours and, until then, while the dielectric substrate voltage level varies between the two DC bias voltage levels, read out errors occur due to scale factor transients as high as 10-15%.
By extending the sense plates on the dielectric substrate and minimizing the amount of exposed dielectric substrate material below the proof masses, the transients were reduced to roughly 1%. Still, for many military and commercial applications, transients as low as 0.01 to 0.1% of final scale factor value are desired.
Possible solutions include commutating the voltage on the pick-off electrode structure, placing AC carrier signals rather than a DC signal on the pick-off electrode structure, placing both AC and DC signals on the sense plate, or controlling the resonant frequency of the drive oscillation to control the drive amplitude.
These potential solutions, however, all suffer from degradation in performance and/or the requirement of costly and complex electronic signal processing circuitry.